"You shall count for yourselves seven complete weeks from the next
day after the Shabbat [i.e. First Day of Pesach]" (Lev. 23:15).

The Torah emphasizes that you shall count for yourselves [in Hebrew,
"lachem"]. This is similar to the commandment requiring a woman
who has her period to count seven days: "Then she shall count for
herself [in Hebrew, "la"] seven days, and after that she shall
be pure." (Lev. 15:28)

Just as she counts for herself [implying for her benefit] so too should
you count for your own benefit. What is the benefit? It is to be purified
in the higher holy waters and afterwards [on Shavuot] to merit to unite
with the King [Zeir Anpin, and to cause the uniting of Zeir Anpin
and Malchut] and to receive the Torah [that emerges as a result of this
union].

Come and see, any person who didn't count this number of seven complete
weeks and thereby earn this purity, is not called "pure" and
is not included amongst the pure and nor is he worthy of receiving his
portion in the Torah. Whoever arrives pure on that day [Shavuot] and remained
conscious of the Counting of the Omer, upon reaching the night [of Shavuot]
ought to learn Torah and unite with it and guard himself in the spiritual
purity that dwells on him that night [by staying awake] and be purified
in it.

And we have learned, a person should learn the Oral Torah on that night
to be purified together [with the Shechina in malchut] from the
flowing of the deep river.
Afterward on that day [at the Musaf/Additional prayer of Shavuot],
the written torah [Zeir Anpin] will come and unite with her and they will
be united as one, above. Then an announcement is made concerning those
who accompanied them saying: "As for Me, this is my covenant with
them says G-d; My spirit that is upon you, and My words that I have put
in your mouth..." (Isaiah 59:21).

[Excerpted from http:/www.KabbalaOnline.org ]

(Chassidic Masters)

[Fom the Baal Shem Tov]

One must be extremely carefeul not to speak a single idle word from the
recital the Tikun of Shavuot night until after the Kedusha prayer of Musaf.
For then, all the Ornaments of the Bride* ascend. Thus, one must be very
careful to purify his thoughts at least until then.

[* These correspond to the 24 books of Scripture that
comprise the main part of the "Tikun" recited on Shavuot night,
as well as the 24 combinations of G-d's holy four-letter name that correspond
to the sephira of Malchus. Thus, by reciting the Tikun on Shavuot night,
one clothes the Bride -the Divine Presence-in beautiful garments that
make her fit for union with the king.]

(Chabad Rebbes)

[From "The Lubavitcher Rebbe's Chumash"]

Ex. 20:1 God then spoke all these words, saying:

In all other instances in the Torah where the word "saying"
(leimor) is used it means that the message is to be conveyed to
a third party. In most cases, it means that Moses is to relate to the
Jewish people what God is telling him. In this case, however, there was
no third party to hear these words later on: every single Jew alive at
the time was at Mount Sinai and heard God say these words. It cannot even
be understood as the obligation to transmit the Torah to later generations,
for the Midrash teaches us that the souls of every Jew that would exist
throughout all of history were present at the giving of the Torah.

Rabbi Akiba and Rabbi Ishmael explain that the word "saying"
in this verse indicates that the Jews responded to each of the Ten Commandments.
According to Rabbi Ishmael, they responded, "Yes [we will do it]"
to the active commandments (such as "Honoring your parents"),
and "No [we will not commit that sin]" to the passive commandments
(such as "Do not steal"). According to Rabbi Akiba, they responded
to both the active and the negative commandments with "Yes!"
meaning "We will do whatever You say."

Their difference of opinion may be explained as follows:
Every mitzvah is an expression of God's will. In this respect all mitzvot
are equal, since they are all equally the will of God. On the other hand,
each mitzvah has its particular, unique effect upon the person performing
it and upon the world. The question of which aspect of the mitzvah should
be paramount in the mind of the person performing it underlies the disagreement
between the two sages. Rabbi Ishmael maintains that the emphasis must
be on the particular aspects of each mitzvah ("yes" to positive
commandments; "no" to negative ones), since the purpose of the
mitzvot is to bring holiness to all the various and different facets of
the individual's life. Rabbi Akiba, in contrast, maintains that the emphasis
must be on the transcendent nature of the mitzvot, i.e., how they express
man's surrender to the will of God ("yes" to everything).

Some Laws and Customs (from Ascent Quarterly)

SUGGESTED
ACTIVITIES for SHAVUOT

1) Switch into high gear!
2) Stay up all night studying Torah to rectify our mistake. 3)
Towards dawn, immerse in a mikvah (or ocean or pool), but don't drive there–it's
Yom Tov! 4) Go to a shul and hear the Ten Declarations. Try to
bring others too–especially Jewish children, for they were our guarentors at the
first Giving of the Torah and they too will benefit by experiencing it now.
Accept the Torah unconditionally with joy and sincerity. 5) Eat some
dairy foods. When we were given the Torah (including the laws of kashrut),
we realized that our cooking vessels were not kosher, so until we kashered
them we ate only dairy products. 6) Read the Book of Ruth: a) King
David, her descendant, died on Shavuot; b) Ruth was a convert and at Sinai we
were like converts –G-d transformed us from ordinary people to a special nation.